NF-κB is a major transcription factor that regulates genes responsible for cell survival, growth and proliferation. T cell receptor (TCR) engagement induces NF-κB activation, which is involved in the host defence against infection and the development of inflammation, cancer and autoimmunity. PKC-θ plays a critical role in IKK-NF-κB activation and T cell function. The adaptor SLP-76 is required for PKC-θ activation upon TCR stimulation. However, the signal transduction from SLP-76 to PKC-θ and the direct kinase activating PKC-θ remain unclear. Thus, the most critical question in TCR-induced NF-κB activation is to identify the pivotal link between SLP-76 and PKC-0.
PKC-θ activation requires its phosphorylation at T538. The kinase PDK1 interacts with PKC-θ and phosphorylation of PKC-θ at T538 is defective in PDK1-deficient T cells. Thus, PDK1 has been proposed to directly phosphorylate PKC-θ at T538, although no clear evidence exists that PDK1 directly phosphorylates PKC-θ in vitro. Furthermore, the observation that PDK1 can be activated only by CD28, but not TCR signaling, further rules out the possibility that PDK1 is the direct kinase for PKC-θ activation induced by TCR signaling. Thus, the kinase that directly activates PKC-θ during T cell activation remains elusive.
GCK-like kinase (GLK; also named MAP4K3) is a member of MAP kinase kinase kinase kinase (MAP4K), which is a subfamily of Ste20-like serine/threonine kinases. GLK contains a conserved N-terminal kinase domain, a conserved C-terminal citron homology domain and several proline-rich motifs in the middle. Jnk phosphorylation is induced by GLK through MEKKI and MKK4/SEK1 in response to stress stimulation. GLK also regulates cell growth via activating the mTOR downstream effectors S6K1 and 4E-BP1 in epithelial cell lines after amino acid treatment. However, the regulatory mechanism and physiological roles of GLK are largely unknown.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies related to regulation of GLK, especially in connection with the physiological role of GLK in the TCR signaling process.